[client] clipboard: add transport provider interface

Route clipboard exchange through a provider interface. Connected
transports take priority while SPICE remains the fallback.

Track asynchronous X11 and Wayland transfers by request, preserving
clipboard ownership across provider changes.

Use PureSpice clipboard status notifications for capability and
lifecycle updates.

client/include/interface/clipboard.h

@@ -0,0 +1,108 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_LG_CLIENT_CLIPBOARD_INTERFACE_
+#define _H_LG_CLIENT_CLIPBOARD_INTERFACE_
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+typedef enum LG_ClipboardData
+{
+  LG_CLIPBOARD_DATA_TEXT = 0,
+  LG_CLIPBOARD_DATA_PNG,
+  LG_CLIPBOARD_DATA_BMP,
+  LG_CLIPBOARD_DATA_TIFF,
+  LG_CLIPBOARD_DATA_JPEG,
+
+  LG_CLIPBOARD_DATA_NONE
+}
+LG_ClipboardData;
+
+typedef void (*LG_ClipboardReplyFn)(void * opaque,
+    LG_ClipboardData type, const uint8_t * data, uint32_t size);
+
+typedef uint64_t LG_ClipboardRequest;
+
+#define LG_CLIPBOARD_REQUEST_INVALID UINT64_C(0)
+
+typedef struct LG_ClipboardStatus
+{
+  bool     available;
+  uint32_t generation;
+}
+LG_ClipboardStatus;
+
+typedef void (*LG_ClipboardStatusFn)(void * opaque,
+    const LG_ClipboardStatus * status);
+
+typedef struct LG_ClipboardEventOps
+{
+  /* The type list and data are borrowed for the duration of the callback.
+   * Event callbacks are always invoked after releasing backend locks. */
+  void (*notice)(void * opaque, const LG_ClipboardData types[], size_t count);
+  void (*data)(void * opaque, LG_ClipboardRequest request,
+      LG_ClipboardData type, const void * data, size_t size);
+  void (*release)(void * opaque);
+  bool (*request)(void * opaque, LG_ClipboardRequest request,
+      LG_ClipboardData type);
+}
+LG_ClipboardEventOps;
+
+typedef struct LG_ClipboardOps
+{
+  const char * name;
+
+  /* Operations must fail safely if the remote endpoint disappears. */
+
+  /* Registration must synchronously report the current status after releasing
+   * any backend locks. Status callbacks must be serialized and generations
+   * must advance whenever availability changes. Passing NULL unregisters the
+   * listener and synchronously quiesces its callbacks. Status callbacks must
+   * not be delivered from another operation or event callback. */
+  void (*setStatusListener)(void * opaque, LG_ClipboardStatusFn callback,
+      void * callbackOpaque);
+
+  /* Attach begins serialized event delivery and may synchronously replay the
+   * current remote notice. Detach synchronously quiesces event callbacks.
+   * Other operations must not deliver event callbacks synchronously. */
+  bool (*attach)(void * opaque, const LG_ClipboardEventOps * events,
+      void * eventOpaque);
+  void (*detach)(void * opaque);
+
+  bool (*release)(void * opaque);
+  /* All outbound arrays and data are borrowed only until the call returns. */
+  bool (*notifyTypes)(void * opaque, const LG_ClipboardData types[],
+      size_t count);
+  /* Data is a complete response to a remote request. NONE with no payload
+   * reports that the request could not be completed. The provider must finish
+   * any transport framing before returning. */
+  bool (*data)(void * opaque, LG_ClipboardRequest request,
+      LG_ClipboardData type, const void * data, size_t size);
+  /* A successful request produces exactly one matching data event unless the
+   * provider is detached, becomes unavailable, or publishes a newer notice
+   * or release first. */
+  bool (*request)(void * opaque, LG_ClipboardRequest request,
+      LG_ClipboardData type);
+}
+LG_ClipboardOps;
+
+#endif